1. Field of the Invention
One aspect of the present invention relates to a method for remediating emissions.
2. Background Art
Environmental regulations regarding emissions from engines and electrical generation stations are in effect in many countries. Among the emissions of regulatory concern are oxides of nitrogen. The oxides of nitrogen include, but are not limited to, nitric oxide, NO, and nitrogen dioxide, NO2. These compounds are frequently referred to as NOx as prescribed by the United States Environmental Protection Agency.
NOx emissions in an emissions stream may occur under lean burn conditions of a diesel engine. In an exhaust system of the diesel engine of a heavy truck, controlling NOx emissions may be a significant challenge due to rapidly changing temperatures and chemical composition of the emissions. Such changes may arise during a rapid acceleration or deceleration of the heavy truck. In such scenario, an inlet emissions temperature of the emissions may change relatively quickly from as low as 130° C. to as high as 600° C. and an air-to-fuel (A/F) ratio of the emissions stream may change relatively quickly from as low as 15 to as high as 100.
Aftertreatment systems have been proposed to remediate NOx in the emissions from a diesel engine. These systems suffer from one or more disadvantages. For example, a lean NOx trap may have difficulty providing relatively good NOx storage capacity at relatively low temperatures of the emissions. By contrast, some lean NOx traps may provide effective NOx remediation techniques for engines with lean exhaust when the inlet emissions temperature is predominantly above 300° C. However, the lean NOx trap may reach capacity with regards to its ability to trap NOx. Restoring the trapping capacity of the lean NOx trap may involve purging. Purging the lean NOx trap may involve providing a fuel-rich emission. Providing the fuel-rich emissions throughout the entire diesel engine and exhaust system may waste expensive fuel.
Another challenge in remediating NOx in diesel systems is matching the concentration of supplied reducing agent to the concentration of NOx in the emissions during transient operations. During transient operations, such as acceleration and deceleration, the exhaust flow rates, temperature of the emissions, and NOx concentrations can change rapidly. Rapid changes in some cases may be controlled by a selective catalytic reduction (SCR) catalyst. The SCR can store NH3, particularly at relatively low temperatures of the emissions. However, at temperatures of 450° C. and above, the SCR effectiveness for controlling NOx emissions in the emissions stream may decrease because the amount of ammonia which the SCR catalyst can store drops to a relatively low level.
In light of the foregoing, what is needed is an effective emissions remediation method for NOx emissions suitable for lean air-to-fuel ratio conditions for a relatively wide range of temperature and NOx concentrations in the emissions.
What is further needed is an emissions remediation method for NOx emissions that avoids wasting expensive fuel.